1. Field of the Invention
The present invention relates generally to a solid-state image pick-up device utilizing a charge coupled device (CCD). More specifically, the invention relates to a solid state image pick-up device which can unify the picture quality at every pixcel.
2. Description of the Prior Art
Solid state image sensors comprising a charge transfer device such as a charge coupled device (hereinafter referred to as a CCD) are classified broadly into the frame transfer type and the interline transfer type. Such solid state image sensors comprising the CCD have been given attention as devices able to realize a compact image pick up apparatus, namely, a television camera in miniaturized size operative with low power consumption and with high reliability. However, contrary to the above advantage, previously proposed solid state image sensors comprising the CCD have encountered several problems with the undesirable phenomena called "blooming" and "smear".
Considering the solid state image sensors of the interline transfer type, such a sensing device comprising a sensing and vertical transfer portion including a plurality of photo-sensing areas provided to make horizontal rows and vertical rows. Vertical charge transfer portions are provided along each of the vertical rows of the photo-sensing areas and transfer gate areas provided between each of the photo-sensing areas and the corresponding one of the vertical charge transfer portions. A horizontal charge transfer portion is coupled with the vertical charge transfer portion and an output portion is coupled with the horizontal charge transfer portion. The sensing and vertical transfer portions, horizontal charge transfer portions and output portions are formed on a common semiconductor substrate. The photo-sensing area is provided for producing a signal charge in response to the light received thereby and storing the signal charge therein. The transfer gate area is provided for transferring the signal charge stored in the photo-sensing area to the vertical charge transfer portion at each period corresponding to a vertical blanking period. The vertical charge transfer portion is provided for transferring the signal charge transferred from the photo-sensing area to the horizontal charge transfer portion in order at every period corresponding to a horizontal blanking period. The horizontal charge transfer portion is provided for transferring the signal charge transferred from the vertical charge transfer portion at each one of the periods corresponding to the horizontal blanking periods to the output portion during a period corresponding to a horizontal video period. Further, the output portion is provided for taking out an image pickup signal output in response to the signal charge transferred from the horizontal charge transfer portion.
In solid state image sensors of the interline transfer type using the CCD (hereinafter referred to as interline transfer CCD image sensors) previously proposed, when the light received by the photo-sensing area reaches the semiconductor substrate under the photo-sensing area through the latter and a charge is produced thereby at the semiconductor substrate, such a charge partially flows into the vertical charge transfer portion undesirably without becoming the signal charge and is undesirably transferred by means of the charge transfer operation of the vertical charge transfer portion. This charge undesirably transferred becomes a noise component in the image pickup signal output derived from the sensor which causes an eyesore of a white line on a picture obtained on an image display apparatus such as a picture tube in response to the image pickup signal output. Such phenomenon as to cause the eyesore of white line on the picture is called "smear" and is one of the unsolved problems encountered with the previously proposed interline transfer CCD image sensors.
Smear on the reproduced picture may also occur due to excessive carriers undesirably overflowing to other photo-sensing areas, when high intensity light is irradiated on the photo-sensing area to generate higher charge than the handling charge of the photo-sensing areas. In order to prevent smear on the picture, a so-called "overflow drain" is preferably provided in the image pick-up device.
In the recent years, a "channel stop region" has been used as the overflow drain for the excessive carrier operated in the photo-sensing areas in interline transfer CCD. The CCD structure with the channel stop region has been illustrated at page 39 of "W. F. Kosonocky, in CHARGE-COUPLED DEVICES: TECHNOLOGY AND APPLICATIONS" published by IEEE (Press, June 1972). In the disclosure, the diffusion channel stop region is formed surrounding the sensing and vertical transfer section, in which the photo-sensing areas are arranged in a form of matrix to constitute a photosensitive array. When the excessive carrier is generated, one of the carrier components, e.g. an electron, is drained to a base layer of the semiconductor substrate, which base layer serves as an overflow drain. The other component, e.g. a hole, of the carrier is then transferred to the channel stop region to be drained. However, an impedance difference at respective photo-sensing areas is caused by a difference in the distance between each photo-sensing area and the channel stop region. This differentiates carrier drain characteristics with respect to each photo-sensing area and thereby differentiates handling charge at an individual photo-sensing area, since the bridgeness of the photo-sensing area is determined depending upon the handling charge thereof. Therefore, due to a difference of the handling charge at different positions of the photo-sensing areas, the brightness of a respective pixel on the reproduced image becomes different. This apparently degrades quality of the picture reproduced.